DREAMZ28

iTrader: (1)

Let's have a discussion.

So HP= torque x rpm / 5252.


I think I've figured out the relationship between acceleration and speed, and how it correlates with horsepower and torque.



CAR A:

350 HP
350 TQ
3500 Raceweight.



CAR B:

425 HP
275 TQ
3500 Raceweight.


CAR C:
275 HP
425 TQ
3500 Raceweight.

Generally, Car B will have a top speed that's higher. That's a given. But will CAR C accelerate harder just because of the higher TQ rating?



What I have been thinking about lately is how horsepower comes from torque.


At higher RPMs, the powerstroke happens more often in a given period of time. The power coming from the engine isn't a constant force, it's repetitive pushes. From the way I understand it, between the time of the power strokes, wind resistance is slowing the car down. Higher RPM= less amount of time between power strokes=less deceleration from wind resistance=higher 'top speed'.

I don't know if I"m on the right track or not, but I'd really like to be able to understand this relation. What do you all think about how I'm looking at this?

JD_AMG

You forgot a major factor: gearing.
Read this link:
http://vettenet.org/torquehp.html

z-maro

iTrader: (18)

Assuming all cars have the same gearing and redline, they would all reach the same top speed (also assuming they have enough power to get there).

HCI2000SS

iTrader: (2)

So many possibilities and factors here, but I think in general the car with broadest hp & tq curve will usually win...all else being equal of course. Being able to carry the torque curve out further is also crucial as well

rickybobbyss

interesting,tq.vs hp.,makes me wonder would my Camaro be faster with a duramax 6.6 in it

DREAMZ28

iTrader: (1)

I'm asking this question with all variables being optimal to each car. Basically with each car running it's best, if car A would run the fastest with 4.56s and car C with 3.73s that's how they're running.

I forgot about the torque curve, but imagine that every car has an equally brood torque curve, if possible.

ayousef

why are you complicating this?

Look in simple temps:

- Torque is the amount of power your engine generates IN ONE REVOLUTION

- Horsepower is the amount of power your engine generates at XXXX amounts of revolutions per minute.

What makes a car faster is horsepower and not torque, ever noticed how hard your car pulls at 6000rpms vs 2500rpms? The correct way to best measure a vehicles acceleration capability is to not use Torque at all and simply convert everything to average HP. The car with the more low-end horsepower will accelerate quicker in this band, the car with mode mid-range horsepower will accelerate harder in midrange and so on so forth.

JD_AMG

Half true.
Engines make torque, and we calculate HP ("work done") from that.
BUT your car's acceleration will match the torque curve exactly. So you call actually pulls hardest at peak torque, not hp.
read:http://vettenet.org/torquehp.html

DREAMZ28

iTrader: (1)

Basically what I'm getting at is if I'm making 350 hp and 290 torque at a certain rpm in one car, and 290 hp and 350 torque at a certain rpm in another car, which would pull harder at those rpms given equal weight? One had to be better than the other... I'm not asking which pulls harder throughout the entire rpm range but which pulls harder at that insurance in time

smokeshow

iTrader: (17)

It all depends on gearing. For equal gearing, those different cars would be substantially different in terms of acceleration. The car with the higher torque would be the clear winner.

"Faster' is subjective. It depends on the context in which you are using that word. The word 'faster' has no physical meaning (and don't take this as me being an ***; I'm just explaining it). Torque however does have a meaning, and it is proportional to the acceleration of the vehicle.

To find instantaneous acceleration of a mass (vehicle):

Torque = force * radius = mass * acceleration * radius

Now, consider a vehicle with a manual transmission in direct drive with a 4.10 final drive ratio and 28" tires. We will assume the inertia of all rotating parts is 0, changes in direction of energy delivery (losses in the rear end) and the radius of the tires is constant (does not change under vehicle weight or torque load).

To find the acceleration of the vehicle, you manipulate that formula to give you acceleration. So, acceleration = torque / (mass * radius). Also incorporate torque multiplication of the rear end. So, for a vehicle producing 350 ft-lbs at 4000rpm, that torque gets multiplied by 4.10. The wheel axle therefore receives 1435 ft-lbs. Now you convert that tire into metric units to give you a radius of 0.3556 meters. Finally, you find the mass of the vehicle, which for a random value for an f-body we can use 3500lbs, or 1588kg. Then, plug all that into the equation:

acceleration = 1435 / (1588 * 0.3556) = 2.541m/s^2

That translates into 0.26G of instantaneous longitudinal acceleration.


Now here is where it gets a bit more complicated. Why do we put camshafts into our vehicles? A cam can net you an increase in torque due to valve lift and several other effects, but the primary purpose for an aftermarket cam is to move the torque curve higher to where you make more horsepower. So, say that same exact car that made 350tq at 4000rpm with an aftermarket camshaft now makes 350tq at 5000rpm. What you have now is more usable RPM which allows for further final drive gear reduction. With the peak torque moved 1000rpms higher in the RPM range, or 25% higher, you would be wise to use a final gear of approximately the same increase. I will use 4.88 gears for the next example. 4.88/4.1 = 1.19, or about 19% higher. This new final drive further increases torque at the wheels without much relative loss in wheel speed. Torque is now 350 * 4.88 = 1708 ft-lbs. Now, use the same equation as before:

acceleration = 1708 / (1588 * 0.3556) = 3.02m/s^2, which translates into 0.31G of instantaneous longitudinal acceleration.

I hope that clears some ambiguity up. If I can explain anything a little better or you want to dig deeper into the calculus part of it where you're not limited to instantaneous calculations, let me know!

NotoriouSS

TQ avarage across the usable RPM range X driveline crawl ratio div by tire height = level playing field compair-o

*at elivated MPH subtract for drag coef.

WHPLSH

This torque statement is not true at all. The only thing your torque curve is a chart of cylinder pressure. Peak torque is peak cylinder pressure. If your cars acceleration followed your torque curve then you would shift shortly after peak torque due to cylinder pressure dropping off. There is a reason you shift your car after peak HP.

The higher the peak torque in the rpm the higher the hp will be.

Here is some math.

350lbs x 3750rpm / 5252 = 250hp

Now take the same peak torque number and move it up 4000rpm

350lbs x 7750rpm / 5252 = 516hp

Torque is pointless, think of it as cylinder pressure and nothing else. horsepower is what wins races. period.

JD_AMG

Nope.
You didn't read the link did you? :http://vettenet.org/torquehp.html
--------------------------------------------------------------------
" First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same.

In contrast to a torque curve (and the matching pushback into your seat), horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver *feels*.

You don't believe all this?

Fine. Take your non turbo car (turbo lag muddles the results) to its torque peak in first gear, and punch it. Notice the belt in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Fine. Can we go on, now? :-) "
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You change gears at peak hp because you want to maximize the amount of TORQUE to the wheels as long as you can (gear ratios manipulate TORQUE, a lower gear = more TORQUE to the wheels - this is why 1st gear pulls harder than 2nd, 2nd harder than 3rd etc etc. 1st gear is a bigger ratio, and that multiples the torque more.

WHPLSH

No. I dont believe it at all. The punching the gas at peak torque then peak hp is dumb and an apples to bananas comparison mostly due to gearing and rpm limits.

The cars acceleration does not follow the torque curve exactly. Look at F1 cars. they make 250 ft lbs peak and 800hp peak. clearly torque is no concern of multi million dollar race teams in F1. If you truly believe torque is what accelerates race cars then go buy a diesel, throw it in a cup car, F1 car or your local drag car. Then find another car with the same peak hp and race it.

If you still think torque decides how your car is going to accelerate then read this article.
http://www.hotrod.com/techarticles/h...e/viewall.html

The Dark Side of Wil

Yes, it does. Exactly.

Say an engine makes 250 ftlbs at 2000 RPM, 300 ftlbs at 3000, 350 at 4000, 400 at 4500 (342 HP), and 350 again at 5500 (366 HP).

With 2.66 first and 3.42 final, axle torque is:
250 - 2274 ftlbs
300 - 2729
350 - 3184
400 - 3638

With a 24" tall tire, this means the tractive effort is:
2274 ftlbs = 2274 lbs tractive effort

2274 ftlbs accelerating a 3000# car makes 0.758 g's of forward acceleration.
You can see that different engine torque results in different tractive effort which results in different acceleration.
Torque translates directly to acceleration.

Torque is how much work is accomplished in 1 revolution of the engine.
Horsepower is the rate at which work is done. That's why power is a function of torque * rpm.

Because the car is moving more slowly at 4000 RPM peak torque than at 5500 RPM peak power, the ftlbs (or WORK, not of torque) per second that the engine is putting into the car are greater at the higher road speed.

JD_AMG

Hmph, went over your head again, and you are still quoting peak numbers as if they are the end all be all...
Lets take your F1 car example.
You say they make a peak of 250ft.lbs, great. Now lets take gearing into account, which as I stated multiplies the torque to the wheels.
Say with a 1st gear ratio of 6.50:1 (relatively speaking, different manufacturers will use different gearing, and none of which will release any information of course), thats a peak of 1625ft.lbs before the final drive gear. And this is pushing a ~1200lbs car. An LS1 6 speed fbody with a stock peak ~375ft.lbs of torque and a first gear ratio of 2.66:1 is making 997ft.lbs before the final drive ratio, and thats pushing a ~3500lbs car... These are peak #s ofcourse, but both cars have a relitivly flat torque curve, which gives them pretty linear acceleration

To say "torque is no concern to a race team" is nothing short of being silly. They spend millions trying to get as much torque at all useable RPMs as possible.

As far as your blind diesel comment, have you checked out LeMans lately?
Audi R10 TDI ring a bell?

More links for ya:
http://www.procivic.com/pages-horsep...que/index.html
http://www.v8914.com/Horsepower-v-torque.htm
http://www.elephantracing.com/techtopic/hpvstorque.htm

usdmholden

I didn't read that vetweb link this time, last time I linked to it I read it (10 years ago?) I thought it was wrong, so I didn't bother reading it this time. Unfortunately, that link has been on the internet for so long, and been wrong for so long, most people don't know what is correct anymore. Just because it's on the internet doesn't mean it's correct.

Torque determines the maximum acceleration which the vehicle can maintain regardless of vehicle velocity.

Power determines the vehicle velocity which the torque can maintain that acceleration.

Gearing increases or decreases torque, but not power.

Torque measured on a chassis dyno is actually calculated back to crank torque, it is not rear wheel torque. I don't know why people never figure this out, but it is true. This is why you always get a chassis dyno readout vs RPM or vehicle speed. The software needs either engine speed or dyno roller speed to figure power.

Once you run out of power, you're done, shift gears. This makes torque mostly irrelevant.

More power and less torque is always faster assuming the vehicle is geared correctly.

JD_AMG

You should probably read it again, its saying what literally every other source is saying (and seems to be what you are saying as well...)
Which is what the link states in a nutshell.

Not really sure where you are going with this... This is common knowledge. The dyno reads torque at the wheels divided by gearing, and you add a 15-20% drivetrain loss to that to get crank torque (and power).

Without torque there is no horse power. If you are talking about torque in the sense of just a figure, sure you can ignore it, but that doesn't change the fact that your cars acceleration exactly matches the torque curve, because torque is what moves the car, thats all im getting at.

Bingo. Because gearing manipulates torque to the wheels.
Although there are some torque curves that are so peaky, it may not make a difference and the lower power car will still be quicker.

ZMX

In a given gear, change in acceleration follows change in the torque curve exactly. If torque suddenly raises by 30%, so does acceleration by 30%.

If downshifting means making the same torque at twice the RPM, then acceleration will be doubled, thanks to gearing multiplication.

So then, engine torque alone does not determine acceleration. Torque and RPM both are required. Torque and RPM combined into a single value are power.

(lb-ft*RPM)/5252 = HP

In any specific gear, acceleration rises and falls with torque. In EVERY gear, kinetic energy is increased by exactly as much power the engine is making.

(1/2)mv^2 = KE

Notice the v^2. Doubling speed quadruples energy. This is why doubling your speed quadruples your braking distance. It’s why a bullet pushes off of a gun to make easily managed recoil on one side and a dead body on the other. It’s why a 200HP car running 16s would need far, far more than 400HP to run 8s.

Acceleration is determined by three things:
1. Power
2. Weight
3. Drag

Top speed is determined by two things:
1. Power
2. Drag

Sometimes, an otherwise identical car with a more powerful engine may be slower than another. This is where the term "dyno queen" came from. Dyno queens with gigantic turbos are often slower than their peak power alone might imply, because they actually make less average power; less "under the curve."


P.S. Bored. Additional crap.
Instantaneous acceleration depends upon:
1. Power put to the ground (at that instant)
2. Current speed
3. Weight
4. Drag

It all comes down to changes in kinetic energy.

Doubling speed quadruples drag - raising energy loss to 8 times as much.
Doubling speed of an object makes it four times as energetic.
Power can be defined as the amount of energy made in a given amount of time.
Change in speed is based on these factors.

miker2

iTrader: (1)

just a little trivia.the mid 50's buick century was pretty peppy for a large ,heavy car to the extent that they were used as police pursuit vehicles.this was with an automatic transmission that was not too kindly called the dynaslush.actually,the dynaflow was a really hi-tech piece.it had drive and low for forward gears.in drive,it did notstart out in low and then shift to high.it remained in high the whole time.so how did the buick engineers quickly accelerate a heavy car without using low gear ? the dynaflow was essentially continuously variable transmission via a variable pitch torque converter.now,the point of all this is that the buick engineers tuned the converter to keep the engine rpm at peak horsepower--not peak torque,while the vehicle accelerated.you stepped on the gas pedal and the revs shot up to a little over 4000 and stayed there while the torque converter continuously changed ratios (stall speeds ).